The impact of climate change on forests is expected to vary globally and regionally. Canadaxxxs Acadian Forest Region lies in the transition between the North American boreal and temperate forest biomes and may be particularly sensitive to changes in climate because many of its component species are currently at their southern or northern climatic range limits. Although some species may be lost, others may exhibit major productivity boosts affecting the goods and services we derive from them. In this study, we use a well-established forest ecosystem simulation model, PICUS, to provide the first exploration of the impact of climate change on the composition and growth of the Acadian Forest Region for the period 2011 to 2100 under two radiative forcing scenarios, RCP 2.6 and RCP 8.5. In the short term (2011-2040), little to no changes in forest composition or growth were projected under either forcing scenario compared with current forest conditions (simulated for 1981-2010 baseline climate); however, by mid-century, PICUS projected increasing departures from the baseline simulations in both composition and growth, with the greatest changes occurring under RCP 8.5 during the late 21st century (2071-2100). Our study indicates that under rapid 21st century warming; Canadaxxxs Acadian Forest Region will begin to lose its boreal character (i.e., "deborealize") as key tree species fail to regenerate and survive. Furthermore, increased growth and establishment by warm -adapted, temperate tree species may be unable to keep pace with the rapid loss of boreal species. This potential "lag effect" may lead to a temporary decrease in forest growth and wood supply during the late 21st century.